The invention relates to high-pressure gas discharge lamps (HID [high intensity discharge] lamps or UHP [ultra high performance] lamps), in particular to mercury high-pressure lamps with mercury fill quantities of between approximately 0.05 and 0.5 mg/mm3, which comprise at least one electrode with an electrode rod, which rod is provided at its end with a thickened, for example globular electrode portion. The invention further relates to a lighting unit with such a high-pressure gas discharge lamp and to a power supply unit for supplying the lamp with operating parameters adapted thereto, as well as to a method of manufacturing the lamp.
The manufacture, the operating characteristics, the service life, and the cost of these lamps are determined to a substantial degree by the nature and shape of the electrodes which are used. Numerous geometric shapes of electrodes have accordingly been developed so as to take into account these criteria in various ways. In the simplest case, the lamp comprises two electrodes which are each formed by a tungsten rod. The free ends of the electrode rods extend into a lamp vessel with a gas atmosphere which renders possible the formation of a light arc in the operational state. The respective other ends are connected to connection pins for receiving an operating voltage via a lead-through extending through the lamp vessel.
The purpose being, for example, to improve the heat radiation of the electrodes and to avoid an excessive heating of the lead-through, and accordingly the risk of damage to the seal at the lamp vessel side at a high lamp power, a known solution is to provide one or several windings from the same material as the electrode at the respective free ends of the electrodes. These windings may possibly be fused to the electrode rod, in particular for achieving the function of a heat buffer in AC-operated lamps. The service life of the electrodes can also be prolonged thereby. Electrodes of this kind can be manufactured comparatively easily from tungsten and are generally known.
An intrinsic disadvantage of these electrodes is, however, that the thermal conductance is usually comparatively low and not reproducible, because the thermal contact between the windings and the rod as well as between individual turns may change during lamp life. These effects may cause changes in the lamp characteristics, i.e. the optical output power and the required operating voltage by up to 30%, in particular in the case of lamps having a short light arc (for example approximately 1 mm). These problems occur substantially independently of whether the windings are fused to the electrode or not, also in the short-arc lamps (for example UHP lamps), because these lamps are operated at such high temperatures (above 3000 K) that also the fused portions are liable to change. Electrodes formed from a suitably strong, solid tungsten rod for the purpose of avoiding this problem are expensive and complicated to manufacture.
An electrode is known from U.S. Pat. No. 3,067,357 in which a tungsten rod has a globular portion created by melting at its free end. The heat required for the melting may be generated during manufacture or during operation of the lamp, the dimensions of said portion, and thus also the electrode spacing, being determined by the lamp current, the pressure inside the lamp, and the diameter of the electrode rod. During operation, a certain proportion (50%) of this portion must always be in the molten state. The manufacture of the electrode should become substantially simpler and less expensive in this manner, because the dimensions of the globular portion, from which the light arc departs, is achieved through a suitable adjustment of said quantities and not through manufacturing and assembling processes which are comparatively sensitive to tolerances, complicated, and expensive.
An essential disadvantage of this lamp is, however, that the lamp current must be adjusted very accurately and must be kept very constant so as to generate the globular portion and to keep it in the molten state in the required proportion. A current which is only a few percents higher may have the result that the entire portion and part of the rod of the electrode melts, so that said portion becomes larger and the distance to the opposite electrode is considerably and permanently changed. This effect is so strong in the case of short light arcs that the current limits must be observed very accurately so as to be able to operate a short-arc lamp with this kind of electrodes in a stable manner. Added to this is that these current limits change during the switch-on phase in dependence on the rising pressure of the gas vapor inside the lamp.
A further disadvantage of this lamp is that the electrode distance increases during lamp life. This is caused basically by the fact that the free iodine atmosphere, by means of which a blackening of the walls is to be prevented, accelerates the transport of tungsten from the hot electrode tip to the rear portions of the electrode. This disadvantage, again, influences short-arc lamps particularly strongly to the extent that they have a maximum life of only a few hundred hours with these electrodes.
It was finally demonstrated that the light arc may periodically move over the front surface of the electrode, in particular in mercury high-pressure lamps (UHP lamps with a pressure of approximately 200 bar) with such an electrode, so that the use of these lamps in projection systems is not possible.
It is accordingly an object of the invention to provide a high-pressure gas discharge lamp of the kind mentioned in the opening paragraph and a lighting unit with such a lamp which during their total operational life provide a stable operation free from fluctuations in combination with a substantially permanent electrode distance, without special requirements having to be imposed on the accuracy and constancy of the lamp current for this.
A further object of the invention is to provide a method whereby such a high-pressure gas discharge lamp can be manufactured in a particularly simple and inexpensive manner.
The former object is achieved according to claim 1, on the one hand, by means of a high-pressure gas discharge lamp of the kind mentioned in the opening paragraph which is characterized in that the thickened electrode portion is dimensioned in dependence on operational parameters of the lamp such that said thickened portion does not melt during normal lamp operation, but that an electrode tip forms itself at the electrode portion during the first hours of operation of the lamp until said tip melts in the region of the point of application of a light arc.
On the other hand, the latter object is achieved according to claim 10 by means of a lighting unit with a high-pressure gas discharge lamp of this kind as well as with a power supply unit for supplying the lamp with operational parameters adapted thereto such that the thickened electrode portion does not melt during normal lamp operation, but that an electrode tip forms itself at the electrode portion during the first hours of operation of the lamp until said tip melts in the region of the point of application of a light arc.
The operational parameters mentioned above are in particular the level of the operating voltage and of the operating current as well as the gradients thereof in time and their frequencies.
The invention is based on the surprising recognition that the electrode tip builds itself up during the first hours of operation of a lamp having such an electrode, which process ends automatically the moment the tip at the end of the electrode starts melting.
A particular advantage of this solution is accordingly that the electrode tip is self-stabilizing as regards its length. A complicated optimization of the electrode spacing is made redundant thereby.
In addition, this self-stabilizing effect remains intact throughout the entire lamp life, so that an optimum electrode distance is present at all times. This advantage is of particular importance for short-arc lamps because the electrodes are highly loaded in these lamps. Furthermore, the lamp is particularly suitable for projection applications because of its stable light arc.
It is true that the electrode tip is molten in the region where the light arc makes contact. However, since the thickened portion has a substantially greater mass as compared with the tip and thus acts as a heat buffer or a heat radiator, the remaining portion of the electrode has substantially lower temperatures, with the result that the lamp has a very long useful life.
To achieve the second object mentioned above, according to claim 7, a method of manufacturing a high-pressure gas discharge lamp is provided which is characterized in that, for the manufacture of the electrode, an electrode rod is provided with a thickened portion at one end, and an electrode tip is formed at this portion during the first hours of operation of the lamp by means of a current which corresponds substantially to the operational current of the lamp, during which the thickened portion is dimensioned in dependence on said current.
The essential advantage of this method is that it is particularly simple and inexpensive, because the usually very complicated manufacture of the electrodes becomes largely redundant, i.e. is limited to the manufacture of the thickened portion at the electrode rod.
The dependent claims relate to advantageous further embodiments of the invention.
The dimensions in accordance with claims 2 and 3 were found to be particularly advantageous for a well-defined formation of the electrode tips.
The embodiments in accordance with claims 4 and 5 have particular advantages as regards the luminous efficacy and the prevention of darkening of the lamp vessel during lamp life.